[BUG][typescript-fetch] Bug when generating typescript-fetch with type: [object, null] together with $ref

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Hello everyone, in our project we need annotate object as nullable. Since we use OpenApi 3.1 where nullable is no more supported. We use this syntax in yaml:

nullableChildObject:
    type:
        - object
        - "null"
    $ref: "#/components/schemas/ChildObject"

and we expect to get code in typescript where nullableChildObject will be defined like:

nullableChildObject: ChildObject | null;

but openapi generator generates definition without null like this:

nullableChildObject: ChildObject;

openapi-generator version

7.20.0 and it didn't work before either, it doesn't work with master too.

OpenAPI declaration file content or url

openapi: 3.1.0
info:
  title: Demo
  description: Just Demo yaml to show issue with type:[object, "null"] and $ref together
  version: 0.0.1
  contact: 
    name: Demo
servers:
  - url: http://localhost
    description: Demo
tags:
  - name: object
    description: Object operations
paths:
  /object:
    get:
      tags:
        - object
      operationId: getObjects
      description: Get objects
      responses:
        "200":
          description: OK
          content:
            "*/*":
              schema:
                type: array
                items:
                  $ref: "#/components/schemas/MainObject"
components:
  schemas:
    ChildObject:
      type: object
      properties:
        someProperty:
          type: string
      required:
        - someProperty
    MainObject:
      type: object
      properties:
        uuid:
          type: string
        nullableProperty:
          type:
            - string
            - "null"
        nullableChildObject:
          type:
            - object
            - "null"
          $ref: "#/components/schemas/ChildObject"
      required:
        - uuid
        - nullableProperty
        - nullableChildObject

Generation Details

java -jar openapi-generator-cli.jar generate --input-spec="api.yaml" --generator-name="typescript-fetch" --template-dir="openapi/" --output="openapi/api" --type-mappings="Date=string" --additional-properties="prefixParameterInterfaces=true,useSingleRequestParameter=false"

Steps to reproduce

Just generate output using yaml and command provided. Than check generated file in openapi/api/models/MainObject.ts if nullableChildObject is defined as union type of ChildObject |null

Related issues/PRs

#21612

Suggest a fix

I have no clue why this doesn't work but I expect that in 3.0 version of openapi, nullable was supported and this null type is new in openapi 3.1. So maybe it just wasn't implemented yet ?

Thank you in advance for any help.

Run the supplied openapi-generator command against the YAML reproduction and inspect openapi/api/models/MainObject.ts. Trace how the typescript-fetch generator handles the type array containing object and null alongside $ref, using related issue #21612 for context. Done when nullableChildObject is generated as ChildObject | null without regressing the existing nullableProperty behavior.